Problem with backlight, or is it something else

My daughter's 4s got wet. I opened it up (thanks to ifixit), cleaned the small amount of corrosion around the connectors near the power switch and inspected for any burn areas. There were none. I also replaced the docking connector as it had suffered with some years of neglect.

Reassembled it, charged the battery and switched it on. Brilliant, it worked, except that after an hour or so the display went blank. Turning off then on produced the same result. After several similar cycles the display stayed off and still remains off. I have since realised that the display is actually on and functioning but only viewable in a strong light.

Classic symptoms of a failed backlight? However, if I do a reset the Apple flashes on as brightly as in the past. Similarly, if the battery is really flat (less that 5%), when I plug in the charger the battery icon comes on to tell me its current state, then goes off.

So how can the backlight not work except at the start of the reset cycle or at the beginning of a charge cycle when the battery is really flat? This infers that the electronics are ok doesn't it?

During the investigations I have stripped the phone several times and restored it to the latest sw with exactly the same results. I am very intrigued and looking for help.

2 Answers

FL4, which connects one of the backlight signals to connector J4, had actually failed. I decided the part was too small for me to replace so I ran a piece of enamelled copper wire from TP22 to C134. This was the easy bit as I then discovered that pin 4 of J4 had fractured. This is the second connection to the backlight. I did think of replacing J4 but again decided the soldering was too much for me without a hot air soldering station. Eventually I ran another wire, this time from TP21 to the display ribbon cable that mates with J4 pin 4. This means that the motherboard is no longer removable without desoldering but it's cheaper than throwing away an iPhone!

So, a bit of a bodge but it all works. Thanks for the comments from others. By the way, if you want schematics of iPhone motherboards just go to 'service-schematics/ru'. Brilliant!

Well done. Just the one! I tested the coil and D1 before I realised that F4 had gone. They were both fine thank goodness. I didn't bother replacing F4 but basically shorted it with the long wire link to C134. All appears good.

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